Simulation Sheds Light on Formation of First Star Clusters

The first star clusters in the universe may have arisen from cold gas clouds nearly 1,000 times more massive than any observed in our galaxy, according to new computer simulations. The findings, presented yesterday at the annual meeting of the Canadian Astronomical Society, also suggest that globular star clusters¿the oldest star clusters in the universe¿were formed in a manner analogous to that of the relatively recent formation of clusters in the Milky Way.

Ralph Pudritz of McMaster University and Melinda Weil of the City College of San Francisco designed their computer models specifically to study the evolution of molecular clouds. Building upon previous work with large-scale models, they essentially applied a magnifying glass to the simulations in order to examine individual galaxy formation. They also accounted for the presence of gas, which many larger models omit because it only accounts for 5 percent of the universe. The researchers determined that supergiant molecular clouds (SGMCs) form when smaller gas clouds present in young galaxies collide and stick together. Using the so-called lambda-CDM cosmological model, which posits that the universe has an accelerating rate of expansion, Pudritz and Weil found that SGMCs attained masses capable of forming globular clusters around 13.2 billion years ago. Because this age matches that of the oldest globular star clusters, the simulation fits well with available observations.

Of particular note is the finding that SGMCs produce mass spectra similar to those of giant molecular clouds (GMCs), which are responsible for the formation of stars in our own galaxy. Thus, the two types of clouds have similar distributions of large objects versus smaller ones. Says Pudritz: "Our findings suggest that the formation of the first star clusters in the universe was not too different in character from the formation of star clusters that we see nearby us in the Milky Way¿such as the famous Trapezium star cluster in the Orion Nebula (see image)." Though they seem to share some similar characteristics, the absolute sizes of the two types of clouds remain significantly different: whereas GMCs have masses close to a million times that of the sun, SGMCs are nearly a billion times as massive as the sun. According to Pudritz, "the Orion star cluster is a dwarf by comparison--thousands of Orion clusters could neatly fit into a globular star cluster."

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